Autoantibody to a cryptic determinant on erythrocytes is predominantly encoded by the germline gene VH11 together with a VKappa9 light chain. Our overall goal is to understand the mechanisms that act to enrich these specificities in CD5+ B cells and determine why such cells are not subject to deletion or anergy as seen in other systems. In the previous funding period we used immunoglobulin transgenic mice to demonstrate ineffective pre-BCR signaling by VH11 1 heavy chain and generated evidence that fetal B cell development is permissive for such pre-BCRs, whereas bone marrow is not. We also found that development of VH11+ B B cells in bone marrow appears further restricted, probably due to the heavy chain's inability to associate with most light chains. Finally, we obtained preliminary evidence of a difference in the capacity of VH11 Vkappa9+ fetal and adult B cells to reach the mature B cell stage. Considering the differences we have observed between high and low copy VH11 transgenic lines, we made a VH11 knock-in mouse to assure physiologic levels of expression. Study of B cell development in this new knock-in, alone or in combination with the prototypic Vkappa9 light chain forms the basis for four of the aims we propose in the next funding period. Specifically, we will characterize fetal and adult B cell development in the VH11 knock-in (VH11T) mouse, alone or in combination with VK9 (Aims 1 and 2). We will also test whether the specificity of the BCR or the gestational stage of its expression determines the entry of VH11vk9+ cells into the CD5+ subset (Aim 3). We will determine whether altering BCR signaling strength and/or sensitivity to BCR signaling can shift VH11 1 B cell fate (Aim 4). Finally, we will ask whether TdT expression is a critical regulator of VH11Vkappa9B 1VK9 B cell generation (Aim 5). All of this work is facilitated by our recent generation of highly specific anti-V region reagents for VH11 1 and Vkappa9. The results of these studies will help to establish the types of selection that shape the expressed B cell repertoire, both fetally and in the adult, with implications for modulating normal and autoimmune responses.